1. Technical Field
This description pertains generally to neurology, and more particularly to electrocardiogram (ECG) signal processing.
2. Background Discussion
Electroencephalography, or EEG, is the recording of electrical activity from the scalp. EEG measures voltage fluctuations that result from ionic current flows within the brain. In clinical contexts, EEG is recorded from multiple electrodes placed on the scalp for periods of minutes to days. Diagnostic applications include detection of normal and abnormal markers of brain function such as observations of brief or extended electrical events that are pathognomonic of certain diseases or of brain and cognitive states. In the clinic, the EEG plays important roles in sleep medicine, epilepsy, brain tumors, anesthesia monitoring, coma, and other serious medical conditions. The analysis of the EEG often includes evaluation of its spectral content of EEG, referring to the common presence of oscillatory components to these signals.
Ballistocardiogram Artifact (BCG) refers to contaminate signals in an electroencephalogram that arise from movement of the body, blood and electric charge across blood vessels due to heart pulsation, particularly when the EEG is recorded within a magnetic field, such as is encountered in a Magnetic Resonance Imaging (MRI) environment.
Electrocardiography (EKG or ECG) is a measure of the heart's electrical activity. Hereafter, the single designation ECG will be used to refer to this signal. ECG is typically a transthoracic (across the thorax or chest) interpretation of the electrical activity of the heart over a period of time, as detected by electrodes attached to the surface of the skin and recorded by a device external to the body. It picks up electrical impulses generated by the polarization and depolarization of cardiac tissue and translates into a waveform. The waveform is then used to measure the timing of heartbeats. The recording produced by this noninvasive procedure is termed an electrocardiogram (ECG).
Under a variety of circumstances, and particularly when EEG signals are acquired during magnetic resonance imaging, the BCG (and at times the ECG) can contaminate the desired EEG signals.
Several methods exist to suppress these artifactual contaminations, but these, in general, depend on accurate timing information for the cardiac-related BCG and ECG signals. Poor quality of the ECG recording will contribute to errors that can occur easily from incorrect placement of the electrodes and/or poor synchronization of the EEG and ECG recordings.
There have been attempts to extract cardiac timing from the BCG. These methods have generally used the mean of the rectified/absolute signal (often referred to as the global field power (GFP)) to emphasize the BCG artifact and extract its timing. However the variance between the timing in this method and the measured cardiac timing often is excessive, leading to poor artifact suppression. As such, the current state of the art for measuring the timing of the BCG events is to infer it based upon the timing of the R-wave complexes of a simultaneously recorded ECG. Where the ECG signal arises from a single source, the change in polarization of the heart muscle, the BCG artifact arises from multiple sources only one of which has a tight time lock to the heartbeat. As a result of the multiple sources of the BCG artifact, there is discrepancy in the timing between ECG signals and some of the components of the BCG artifact. This leads ultimately to poor artifact rejection and degraded EEG.